The identification or production of microbial strains with enhanced biological activity, e.g., strains with enhanced nitrogen fixation capability, or enhanced pesticidal activity, is an important goal in agricultural biotechnology. In many agricultural applications an improved microbial strain is added to the soil and thus must compete with native free-living microbes for growth and nutrients. Even though a microbe possesses a useful biological activity, failure to compete with native organisms will most often render it ineffective in the field. For example, displacement of established populations of Bradyrhizobium japonicum by inoculation with strains that are efficient nitrogen fixers has proven difficult (Triplett (1990) Molec. Plant Microb. Interactions 3:199-206). Efforts to improve the competitiveness of specific microorganisms in agricultural settings have included searches for naturally occurring strains which possess a competitive advantage, Tn5 mutagenesis-selection experiments, and experiments designed to exploit the effects of various naturally occurring indigenous plasmids.